Live bacterial vaccines and their application as carriers for foreign antigens

NALT M cells are important for immune induction for the common mucosal immune system

Nasopharynx-associated lymphoid tissue (NALT) is one of the major constituents of the mucosa-associated lymphoid tissue (MALT), and has the ability to induce antigen-specific immune responses. However, the molecular mechanisms responsible for antigen uptake from the nasal cavity into the NALT remain largely unknown. Immunohistochemical analysis showed that CCL9 and CCL20 were co-localized with glycoprotein 2 (GP2) in the epithelium covering NALT, suggesting the existence of M cells in NALT. In analogy with the reduced number of Peyer's patch M cells in CCR6-deficient mice, the number of NALT M cells was drastically decreased in CCR6-deficient mice compared with the wild-type mice. Translocation of nasally administered Salmonella enterica serovar Typhimurium into NALT via NALT M cells was impaired in CCR6-deficient mice, whereas S. Typhimurium demonstrated consistent co-localization with NALT M cells in wild-type mice. When wild-type mice were nasally administered with an attenuated vaccine strain of S. Typhimurium, the mice were protected from a subsequent challenge with wild-type S. Typhimurium. Antigen-specific fecal and nasal IgA was detected after nasal immunization with the attenuated vaccine strain of S. Typhimurium only in wild-type mice but not in CCR6-deficient mice. Taken together, these observations demonstrate that NALT M cells are important as a first line of defense against infection by enabling activation of the common mucosal immune system (CMIS).

An aromatic amino acid auxotrophic mutant of Bordetella bronchiseptica is attenuated and immunogenic in a mouse model of infection

We have constructed an aromatic amino acid auxotrophic mutant of Bordetella bronchiseptica, harbouring mutations in aroA and trpE to investigate the use of such a strain as a live-attenuated vaccine. B. bronchiseptica aroA trpE was unable to grow in minimal medium without aromatic supplementation. Compared to the parental wild-type strain, the mutant displayed significantly reduced abilities to invade and survive within the mouse macrophage-like cell line J774A.1 in vitro and in the murine respiratory tract following experimental intranasal infection. Mice vaccinated with B. bronchiseptica aroA trpE displayed significant dose-dependent increases in B. bronchiseptica-specific antibody responses, and exhibited increases in the number of B. bronchiseptica-reactive spleen cells in lymphoproliferation assays. Immunised animals were protected against lung colonisation after challenge with the wild-type parental strain. With such a broad host range displayed by B. bronchiseptica, the attenuated strain constructed in this study may not only be used for the prevention of B. bronchiseptica-associated disease, but also for the potential delivery of heterologous antigen.

Oral immunization of swine with attenuated Salmonella typhimurium aroA SL3261 expressing a recombinant antigen of Mycoplasma hyopneumoniae (NrdF) primes the immune system for a NrdF specific secretory IgA response in the lungs

Salmonella typhimurium SL3261 (aroA mutant) expressing a recombinant Mycoplasma hyopneumoniae antigen was used to orally immunize swine against porcine enzootic pneumonia. This construct, designated S. typhimurium aro A SL3261 (pKF1), expressed a recombinant protein containing the carboxy-terminal 11 kDa of a 42 kDa M. hyopneumoniae NrdF ribonucleotide reductase R2 subunit protein. Here we demonstrate that this antigen is present in all seven geographically diverse strains of M. hyopneumoniae tested, and is recognized by the swine immune system after experimental infection with the virulent M. hyopneumoniae Beaufort strain. The immune response of swine orally immunized twice with S. typhimurium SL3261 (pKF1) on day 0 and day 14 was evaluated. Oral immunization with S. typhimurium SL3261 (pKF1) primed the immune system to elicit a significant (P<0.05) secretory IgA response against the 15 kDa NrdF antigen in the respiratory tract of swine, post-challenge, compared to control groups. Blood lymphocytes from swine immunized with S. typhimurium SL3261 (pKF1) proliferated significantly (P<0.05) following stimulation with M. hyopneumoniae whole-cell extracts compared to control groups 14 days post-vaccination. Following challenge with virulent M. hyopneumoniae, swine immunized with S. typhimurium SL3261 (pKF1) showed higher average daily weight gains and reduced lung pathology compared to control groups.

Potential live vaccines for HIV

Potential live vaccines for HIV were developed using an Lpp-OmpA system to target an HIV antigen, reverse transcriptase, or an immunodominant epitope of this enzyme, to the outer membrane of an attenuated strain of Salmonella SL3261. These live vaccines were administered orally to mice, and fecal IgA and helper T cell responses were measured. Results indicated a fecal IgA response specific to HIV reverse transcriptase, as well as a reverse transcriptase-specific helper T cell response, as measured by proliferation assays. Additionally, tests with the epitope vaccines showed a selective cytotoxic CD8 T cell response. These results suggest that this method of antigen targeting to the outer membrane of attenuated bacterial vectors is very promising not only for HIV vaccine development, but also for antigens from other viral or bacterial pathogens, which could be inserted into the Lpp-OmpA protein construct, to elicit mucosal IgA and T cell responses.

Recombinant, attenuated Salmonella typhimurium stimulate lymphoproliferative responses to SIV capsid antigen in rhesus macaques

Recombinant bacteria are useful vectors for delivering foreign antigens to mucosal surfaces and may elicit immune protection against sexually-transmitted pathogens. Recombinant, attenuated Salmonella typhimurium expressing the Simian Immunodeficiency Virus capsid protein (p27) were given to rhesus macaques by intragastric intubation. This route of immunization was compared with intramuscular injection of soluble p27 in adjuvant, and with immunization protocols that combined intragastric and intramuscular antigen exposures. Recombinant Salmonella stimulated p27-specific lymphoproliferative responses that were present transiently in peripheral blood, and were recalled easily by booster immunizations. Intramuscular p27 injection elicited strong serum antibody responses, but only low level capsid-specific proliferative responses. Recombinant Salmonella immunization elicited low levels of p27-specific antibodies in serum and did not suppress subsequent responses to parenteral immunization. Intragastric immunization of macaques with recombinant Salmonella typhimurium was safe and induced immune responses specific for the expressed, foreign antigen.

Immunization of Macaca mulatta with aroA attenuated Salmonella typhimurium expressing the SIVp27 antigen

Attenuated bacteria expressing foreign antigens stimulate both systemic and mucosal immune responses to the recombinant protein. We studied the infection of rhesus macaques with an attenuated Salmonella typhimurium expressing the simian immunodeficiency virus p27 capsid protein. Juvenile rhesus macaques were inoculated by intragastric intubation with doses ranging from 3 to 9 x 10(9) viable aroA attenuated S. typhimurium. The bacterial infection was self-limiting with no overt clinical signs. Salmonella were shed in the feces of macaques for approximately five days. Salmonella were isolated from fecal material to examine the in vivo stability of both the attenuating mutation and the integrated SIVp27 expression cassette. All Salmonella isolates retained both the attenuating mutation and the recombinant expression construct. In vitro analysis showed that a minimum of 7.2 microg of p27 was delivered by a single oral dose with attenuated, recombinant S. typhimurium.

Vaccination against animal parasites

A decade of molecular parasitology is beginning to bear fruit, with the appearance of several new, highly effective, practical vaccines against parasitic diseases. Recombinant antigen vaccines have been developed against cestode, nematode, trematode, protozoan and arthropod parasites. Greatest progress has been made with veterinary vaccines, where the ability to test numerous vaccine formulations in challenge trials has allowed more rapid identification of host-protective antigens than is possible with many medically important parasites. Several quite different approaches to vaccine development have been successful. The traditional approach using live, attenuated parasites continues to provide effective vaccines against several protozoan and nematode parasites. Recombinant DNA technology, monoclonal antibody technology, protein chemistry and immunochemistry have played critical roles in the outstanding success which has been achieved over the last 5 years in the development of defined-antigen vaccines. Two approaches have been successful in research towards defined antigen vaccines against parasites: (1) the 'natural antigen' approach where immune responses are stimulated to parasite molecules which are normally antigenic, and possibly host-protective, in infected hosts; (2) the 'naive antigen' approach where parasite molecules which are not antigenic, or of very low antigenicity, in infected hosts are used to raise immune responses capable of killing the parasite. This review examines the successful approaches taken towards the development of effective anti-parasite vaccines and the vaccines which have been produced to date.

Swine immunity to an attenuated Salmonella typhimurium mutant containing a recombinant plasmid which codes for production of a 31-kilodalton protein of Brucella abortus

Salmonella typhimurium chi 4064, an attenuated delta cya delta crp mutant of S. typhimurium SR-11, was shown to be avirulent in swine. S. typhimurium chi 4064 was used as a carrier for plasmid pBA31-R7, which codes for the expression of a 31-kDa protein from Brucella abortus (BCSP31). Given orally, S. typhimurium chi 4064(pBA31-R7) colonized the intestine and mesenteric lymph nodes of 5- to 6-week-old crossbred swine. Orally immunized animals developed serum and intestinal antibody responses to the B. abortus 31-kDa protein and to salmonella endotoxin as measured by enzyme-linked immunosorbent assay. Similarly immunized swine did not develop delayed-type hypersensitivity following a subcutaneous injection of recombinant BCSP31. However, swine parenterally immunized with recombinant BCSP31 incorporated in Freund incomplete adjuvant did develop a delayed-type hypersensitivity response to the homologous antigen. The data indicated that oral presentation of antigen to swine in the context of recombinant S. typhimurium effective stimulated mucosal and systemic antibody-mediated immunity but failed to sensitize swine for either an antigen-specific delayed-type hypersensitivity or a blastogenic response to the cloned BCSP31.

Schistosome vaccines

Schistosomiasis control currently relies primarily on chemotherapy which is both expensive and temporary. There is an urgent need for an effective vaccine. Studies in animal models and man have demonstrated the existence of protective immunity. Antibody-dependent cell-mediated cytotoxicity mechanisms involving eosinophils and macrophages have been implemented in destruction of the parasites. Antigens expressed on the surface of the schistosomulum are among the targets of protective immune responses. Vaccines comprising recombinant antigens are now being tested in vivo for their capacity to evoke protective responses. Live oral vaccines based on attenuated Salmonella expressing schistosomular surface antigens are being developed.

The role of a stress-response protein in Salmonella typhimurium virulence

We recently described the use of selective transposon mutagenesis to generate a series of avirulent mutants of a pathogenic strain of Salmonella typhimurium. Cloning and sequencing of the insertion sites from two of these mutants reveals that both have identical locations within an open reading frame that is highly homologous to a gene, htrA, encoding a heat-shock protein in Escherichia coli. DNA sequence analysis of S. typhimurium htrA reveals the presence of a gene capable of encoding a protein with a calculated Mr of 49316 that has 88.7% protein:protein homology with its E. coli counterpart. In E. coli, lesions in this gene, also known as degP, reduce proteolytic degradation of aberrant periplasmic proteins. Characteristics of the S. typhimurium htrA mutants, 046 and 014, in vivo and in vitro suggested that they are avirulent because of impaired ability to survive and/or replicate in host tissues. In vitro, the S. typhimurium htrA mutants 046 and 014 are not temperature-sensitive but were found to be more susceptible to oxidative stress than the parent, suggesting that they may be less able to withstand oxidative killing within macrophages.

Immunity conferred by Aro- Salmonella live vaccines

The specificity of protection conferred by Aro- salmonellae was studied in BALB/c mice challenged 3 months after intravenous (i.v.) vaccination, more than 1 month after the vaccine had been cleared. Oral challenge showed better protection than i.v. challenge. Salmonella typhimurium aroA SL3261 conferred very good protection against wild-type S. typhimurium C5 (over 10,000 x LD50). Cross protection experiments were performed using S. typhimurium, S. enteritidis and S. dublin for vaccination and challenge, including variants of S. typhimurium and S. enteritidis of similar virulence differing in the main LPS antigen (O-4 or O-9). Salmonella typhimurium aroA conferred solid protection against S. typhimurium (O-4), but no protection against wild-type S. enteritidis (O-9). However challenge with LPS variant strains showed that although protection was generally better to strains of the homologous LPS type, specificity of protection was determined more by the parent strain background (S. typhimurium or S. enteritidis) of the challenge than by O-factors 4 or 9, suggesting that other antigens are involved. The nature of the protective antigen(s) in this model is unclear, but it does not appear to be the main O-specific antigen. A S. enteritidis Se795 aroA vaccine gave good protection against wild-type S. enteritidis Se795 2 weeks after vaccination, but much less at 3 months (approximately 10-200 x LD50), although the persistence of the S. enteritidis aroA vaccine in the liver and spleen was similar to that of the S. typhimurium vaccine, and the wild-type Se795 challenge strain was of similar virulence to S. typhimurium C5. A S. dublin aroA vaccine conferred similar protection against wild-type S. dublin (approximately 300 x LD50).

Immunity induced by live attenuated Salmonella vaccines

Studies on the degree and specificity of protection conferred by immunization with aroA salmonella live vaccines in BALB/c mice are described. Animals were immunized i.v. and challenged orally 3 months later to ensure that the vaccine had been cleared from the tissues. Vaccination with Salmonella typhimurium aroA SL3261 conferred very good protection against virulent S. typhimurium C5 (over 10,000 x LD50). The specificity of cross protection was studied using S. typhimurium, Salmonella enteritidis and Salmonella dublin for vaccination and challenge, including challenge with variants of S. typhimurium and S. enteritidis of similar virulence which differed in the main LPS (lipopolysaccharide) antigen (0-4 or 0-9). S. typhimurium SL3261 gave very good protection against S. typhimurium C5 (0-4), but no protection against S. enteritidis Se795 (0-9). However, challenge with strains differing in the main 0 antigens showed that, although protection was generally better to strains expressing the same LPS type as the vaccine, specificity of protection was determined more by the background (S. typhimurium or S. enteritidis) of the parent strain used for the challenge than by 0 factors 4 or 9, suggesting that other factors could be involved. The nature of the antigen(s) responsible for protection in this model is unclear, but it would not appear to be the main 0-specific antigen. An S. enteritidis Se795 aroA vaccine was far less effective than S. typhimurium SL3261; it conferred good protection against the homologous wild type at 2 weeks post-vaccination, but far less at three months (approx 10-200 x LD50). This was unexpected, as the persistence of the S. enteritidis vaccine in the liver and spleen was similar to that of S. typhimurium SL3261, and the S. enteritidis and S. typhimurium challenge strains were of similar virulence. An S. dublin aroA vaccine conferred similar protection against wild type S. dublin (approx 300 x LD50).

Alternative methods of attenuating Salmonella species for potential vaccine use

Because of their invasive properties, Salmonella species are uniquely suited for use as live oral vaccines, and mutants attenuated in general metabolic functions show promise as vaccine strains. We have identified a Salmonella choleraesuis transposon mutant which is deficient in abilities to attach to, invade or penetrate through animal cells. This mutant is avirulent in mice, yet is able to persist within the liver, spleen and gut-associated lymphoid tissue for periods which may be adequate for induction of secretory, humoral and cell-mediated immunity. This and other strains attenuated in factors specifically required for Salmonella pathogenesis may have potential as vaccine strains.

Moderate immunodeficiency does not increase susceptibility to Salmonella typhimurium aroA live vaccines in mice

Salmonellae carrying appropriate mutations in genes of the aromatic biosynthesis pathway are effective as live vaccines in animals, and they are candidate typhoid vaccines for human use. They are also very effective as carriers of recombinant antigens from other pathogens to the immune system, eliciting circulatory, secretory, and cell-mediated immunity to foreign antigens. Their attenuation is believed to be due to their requirement for the metabolites p-aminobenzoic acid and 2,3-dihydroxybenzoate, which are not available in mammalian tissues. Immunosuppression (e.g., acquired immunodeficiency syndrome) is a major contraindication to the use of live vaccines. If the avirulence of Aro mutants is largely due to their auxotrophy, they should not be markedly more invasive in immunosuppressed animals. We report that wild-type Salmonella typhimurium M525 of intermediate virulence was much more invasive in sublethally irradiated BALB/c mice than in normal BALB/c mice, whereas sublethal irradiation had little if any effect on the invasiveness of an S. typhimurium aorA vaccine strain apart from a delay in its clearance from the reticuloendothelial system. xid mutant CBA/N mice carry an X-linked B-cell functional defect which results in immunoglobulin G3 agammaglobulinemia, and they are known to be more susceptible to salmonellae in late stages of the infection. We found that whereas male (CBA/N x BALB/c)F1 mice (immunodefective) were more susceptible to wild-type S. typhimurium C5 than female littermates (immunocompetent), there was no difference in the response to the S. typhimurium aroA vaccine strain. The results indicate that moderate immunosuppression does not markedly enhance susceptibility to S. typhimurium aroA live vaccines.